The present invention relates to a method of refining aluminum. More particularly, it relates to a method of refining a crude aluminum alloy refined by a blast furnace method.
Aluminum is the most basic material next to iron and the demand for it has been increasingly made year after year.
Up to this time, aluminum has been refined by the electrolysis of molten alumina.
Recently, the refining of alumina has become more difficult with increasing energy cost on a worldwide scale, especially in a region in which the cost of electric power is high, for example, in Japan.
The inventors of the present invention proposed, as a substitute for the above electrolysis method, a method of refining aluminum by using a blast furnace which comprises reducing alumina as a raw material with a carbon material in a blast furnace of a countercurrent moving bed (see U.S. Pat. No. 4,445,934).
According to this blast furnace method, aluminum is obtained as a crude alloy containing iron and silicon. This crude alloy is extracted with a molten metal such as lead or magnesium, and the molten metal is separated from the extract by distillation or a liquid separation method. The residue is further refined by distillation or crystallization to obtain a usable aluminum.
However, the aluminum alloy obtained by this blast furnace method contains a large amount of impurities such as carbon and calcium carbide together with slag components such as calcia and alumina, so that these impurities hinder the contact between the molten extractant metal and aluminum in the above extraction step, thus decreasing the extraction efficiency of aluminum. Further, these impurities contaminate the extractant metals such as lead and magnesium to decrease the recovery of these metals.
Furthermore, it is impossible to separate the slag, carbon and calcium carbide are separated from the crude alloy in a molten state beforehand to avoid adverse influences of these impurities, because the difference in specific gravity between aluminum and the slag or the impurities is so small that these impurities and molten aluminum are dispersed in each other.
The addition of a flux such as calcium fluoride or cryolite hardly changed the above state.